spec: Fix run-time panic header id

Everything was linking to "Run_time_panics", but the id was "Run_time_errors".

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/961042

doc/go_spec.html

@@ -4972,7 +4972,7 @@ Implementation restriction: The compiler assumes package <code>main</code>
 is not imported by any other package.
 </p>
 
-<h2 id="Run_time_errors">Run-time panics</h2>
+<h2 id="Run_time_panics">Run-time panics</h2>
 
 <p>
 Execution errors such as attempting to index an array out