[patch][wwwdocs] Fixed incorrect copy paste in codingconventions.html

[Chris Gregory]

In the `Extern "C"` commentary, the coding conventions said:

    Definitions within the body of a namespace are not indented.

Now it reads:

    Definitions within the body of an <code>extern "C"</code> block
are not indented.

Initially reported at:
https://gcc.gnu.org/ml/gcc/2016-04/msg00211.html

Cheers,

Chris Gregory!

Index: htdocs/codingconventions.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/codingconventions.html,v
retrieving revision 1.73
diff -r1.73 codingconventions.html
1306c1306
< Definitions within the body of a namespace are not indented.
---
> Definitions within the body of an <code>extern "C"</code> block are not 
> indented.