docs: make "runtime" a word only as a name for the package.

Computer people have an agglutinating streak that I like to resist.
As a time of execution: run time.
As an adjective: run-time.
As a noun: run-time support/code/library.

Signed,
Mr. Pedant.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4252048
diff --git a/doc/go_for_cpp_programmers.html b/doc/go_for_cpp_programmers.html
index 608ab14..7168f1d 100644
--- a/doc/go_for_cpp_programmers.html
+++ b/doc/go_for_cpp_programmers.html
@@ -555,7 +555,7 @@
 A variable which has an interface type may be converted to have a
 different interface type using a special construct called a type assertion.
 This is implemented dynamically
-at runtime, like C++ <code>dynamic_cast</code>.  Unlike
+at run time, like C++ <code>dynamic_cast</code>.  Unlike
 <code>dynamic_cast</code>, there does
 not need to be any declared relationship between the two interfaces.
 
@@ -589,7 +589,7 @@
 values of the contained type.  As the typing is dynamic rather
 than static, there is no equivalent of the way that a C++ template may
 inline the relevant operations.  The operations are fully type-checked
-at runtime, but all operations will involve a function call.
+at run time, but all operations will involve a function call.
 
 <pre>
 type iterator interface {