doc/effective_go.html: undo local font change

R=r
CC=golang-dev
https://golang.org/cl/5876043

doc/effective_go.html

@@ -2704,7 +2704,7 @@ unknown format is "image: unknown format".
 <p>
 Callers that care about the precise error details can
 use a type switch or a type assertion to look for specific
-errors and extract details.  For <code>PathError</code>s
+errors and extract details.  For <code>PathErrors</code>
 this might include examining the internal <code>Err</code>
 field for recoverable failures.
 </p>