docs: standardize Markdown headings

examples/mrbgems/c_and_ruby_extension_example/README.md

@@ -1,4 +1,3 @@
-C and Ruby Extension Example
-=========
+# C and Ruby Extension Example
 
 This is an example gem which implements a C and Ruby extension.

examples/mrbgems/c_extension_example/README.md

@@ -1,4 +1,3 @@
-C Extension Example
-=========
+# C Extension Example
 
 This is an example gem which implements a C extension.

examples/mrbgems/ruby_extension_example/README.md

@@ -1,4 +1,3 @@
-Pure Ruby Extension Example
-=========
+# Pure Ruby Extension Example
 
 This is an example gem which implements a pure Ruby extension.

mrbgems/mruby-test/README.md

@@ -1,5 +1,4 @@
-Running Tests
-=============
+# Running Tests
 
 To run the tests, execute the following from the project's root directory.