docs: fix typo in section closing comments in math/base/special/gammasgn* READMEs

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---

Author: Planeshifter

lib/node_modules/@stdlib/math/base/special/gammasgn/README.md

@@ -78,7 +78,7 @@ v = gammasgn( NaN );
 
 </section>
 
-<!-- /. notes -->
+<!-- /.notes -->
 
 <section class="examples">
 

lib/node_modules/@stdlib/math/base/special/gammasgnf/README.md

@@ -78,7 +78,7 @@ v = gammasgnf( NaN );
 
 </section>
 
-<!-- /. notes -->
+<!-- /.notes -->
 
 <section class="examples">