Fix typo in ImportC article (#3081)

Author: drug007

spec/importc.dd

@@ -18,7 +18,7 @@ $(HEADERNAV_TOC)
     It describes the specifics of the dialect of C that
     ImportC is, and how to use it effectively.)
 
-    $(P ImportC and $(LINK2 https://dlang.org/spec/betterc.html) are very different.
+    $(P ImportC and $(LINK2 https://dlang.org/spec/betterc.html, BetterC) are very different.
     ImportC is an actual C compiler. BetterC is a subset of D that relies only on the
     existence of the C Standard library.)