message: Fix broken link syntax in documentation
  The links for Zipkin propagation are not displaying correctly on the HTTP
  header manipulation page in the documentation. This commit fixes a small
  syntax error, allowing these links to be rendered as proper hyperlinks.
diff: @@ -378,7 +378,7 @@ x-b3-traceid
  The *x-b3-traceid* HTTP header is used by the Zipkin tracer in Envoy.
  The TraceId is 64-bit in length and indicates the overall ID of the
  trace. Every span in a trace shares this ID. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation>`.
  +`here <https://github.com/openzipkin/b3-propagation>`__.
  .. _config_http_conn_man_headers_x-b3-spanid:
  @@ -389,7 +389,7 @@ The *x-b3-spanid* HTTP header is used by the Zipkin tracer in Envoy.
  The SpanId is 64-bit in length and indicates the position of the current
  operation in the trace tree. The value should not be interpreted: it may or
  may not be derived from the value of the TraceId. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation>`.
  +`here <https://github.com/openzipkin/b3-propagation>`__.
  .. _config_http_conn_man_headers_x-b3-parentspanid:
  @@ -400,7 +400,7 @@ The *x-b3-parentspanid* HTTP header is used by the Zipkin tracer in Envoy.
  The ParentSpanId is 64-bit in length and indicates the position of the
  parent operation in the trace tree. When the span is the root of the trace
  tree, the ParentSpanId is absent. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation>`.
  +`here <https://github.com/openzipkin/b3-propagation>`__.
  .. _config_http_conn_man_headers_x-b3-sampled:
  @@ -411,7 +411,7 @@ The *x-b3-sampled* HTTP header is used by the Zipkin tracer in Envoy.
  When the Sampled flag is either not specified or set to 1, the span will be reported to the tracing
  system. Once Sampled is set to 0 or 1, the same
  value should be consistently sent downstream. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation>`.
  +`here <https://github.com/openzipkin/b3-propagation>`__.
  .. _config_http_conn_man_headers_x-b3-flags:
  @@ -421,7 +421,7 @@ x-b3-flags
  The *x-b3-flags* HTTP header is used by the Zipkin tracer in Envoy.
  The encode one or more options. For example, Debug is encoded as
  ``X-B3-Flags: 1``. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation>`.
  +`here <https://github.com/openzipkin/b3-propagation>`__.
  .. _config_http_conn_man_headers_b3:
  @@ -430,7 +430,7 @@ b3
  The *b3* HTTP header is used by the Zipkin tracer in Envoy.
  Is a more compressed header format. See more on zipkin tracing
  -`here <https://github.com/openzipkin/b3-propagation#single-header>`.
  +`here <https://github.com/openzipkin/b3-propagation#single-header>`__.
  .. _config_http_conn_man_headers_x-datadog-trace-id:
  