doc: unify colons and spacing in diagnostics guide

Unify colons (outside of <strong></strong>) and add a missing space in
the list of groups of diagnostics solutions.

Change-Id: Icbcd94427d4905dd88c4ea82aaa5dbf064c00990
Reviewed-on: https://go-review.googlesource.com/63111Reviewed-by: JBD <jbd@google.com>

doc/diagnostics.html

@@ -17,10 +17,10 @@ Diagnostics solutions can be categorized into the following groups:
 </p>
 
 <ul>
-<li><strong>Profiling</strong> Profiling tools analyze the complexity and costs of a
+<li><strong>Profiling</strong>: Profiling tools analyze the complexity and costs of a
 Go program such as its memory usage and frequently called
 functions to identify the expensive sections of a Go program.</li>
-<li><strong>Tracing:</strong>Tracing is a way to instrument code to analyze latency
+<li><strong>Tracing</strong>: Tracing is a way to instrument code to analyze latency
 throughout the lifecycle of a call or user request. Traces provide an
 overview of how much latency each component contributes to the overall
 latency in a system. Traces can span multiple Go processes.</li>