docs: standardize Markdown headings
MODIFY
examples/mrbgems/c_and_ruby_extension_example/README.md
examples/mrbgems/c_and_ruby_extension_example/README.md
-C and Ruby Extension Example
-=========
+# C and Ruby Extension Example
This is an example gem which implements a C and Ruby extension.

MODIFY
examples/mrbgems/c_extension_example/README.md
examples/mrbgems/c_extension_example/README.md
-C Extension Example
-=========
+# C Extension Example
This is an example gem which implements a C extension.

MODIFY
examples/mrbgems/ruby_extension_example/README.md
examples/mrbgems/ruby_extension_example/README.md
-Pure Ruby Extension Example
-=========
+# Pure Ruby Extension Example
This is an example gem which implements a pure Ruby extension.

MODIFY
mrbgems/mruby-test/README.md
mrbgems/mruby-test/README.md
-Running Tests
-=============
+# Running Tests
To run the tests, execute the following from the project's root directory.

docs: standardize Markdown headings