doc: correct some minor HTML errors found by tidy

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7439046

doc/contribute.html

@@ -293,7 +293,7 @@ run <code>hg change 99999</code>.
 
 <p>
 Alternatively, you can use
-<p/>
+</p>
 
 <pre>
 $ hg file 99999 somefile

doc/debugging_with_gdb.html

@@ -179,7 +179,7 @@ $ <b>gdb regexp.test -d $GOROOT</b>
 If for some reason GDB still can't find that directory or that script, you can load
 it by hand by telling gdb (assuming you have the go sources in
 <code>~/go/</code>):
-<p>
+</p>
 
 <pre>
 (gdb) <b>source ~/go/src/pkg/runtime/runtime-gdb.py</b>

doc/go1.1.html

@@ -131,7 +131,7 @@ to convert the code to Go 1.0 first.
 
 <h2 id="library">Changes to the standard library</h2>
 
-<h3 id="debug/elf">debug/elf</h3>
+<h3 id="debug_elf">debug/elf</h3>
 <p>
 Previous versions of the debug/elf package intentionally skipped over the first
 symbol in the ELF symbol table, since it is always an empty symbol. This symbol
@@ -141,7 +141,7 @@ calls the debug/elf functions Symbols or ImportedSymbols may need to be
 adjusted to account for the additional symbol and the change in symbol offsets.
 </p>
 
-<h3 id="html/template">html/template</h3>
+<h3 id="html_template">html/template</h3>
 
 <p>
 Templates using the undocumented and only partially implemented

doc/go_spec.html

@@ -2637,7 +2637,7 @@ a[:]   // same as a[0 : len(a)]
 
 <p>
 For arrays or strings, the indices <code>low</code> and <code>high</code> are
-<i>in range</i> if <code>0 &lt;= <code>low</code> &lt;= <code>high</code> &lt;= len(a)</code>,
+<i>in range</i> if <code>0</code> &lt;= <code>low</code> &lt;= <code>high</code> &lt;= <code>len(a)</code>,
 otherwise they are <i>out of range</i>.
 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
 A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type