Correct ^0 asciidoc syntax in fast-import docs.

I wrote this documentation with asciidoc 7.1.2, but apparently
asciidoc 8 assumes ^ means superscript.  The solution was already
documented in rev-parse's manpage and is to use {caret} instead.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-fast-import.txt

@@ -380,9 +380,9 @@ current branch value should be written as:
 ----
 	from refs/heads/branch^0
 ----
-The `^0` suffix is necessary as gfi does not permit a branch to
+The `{caret}0` suffix is necessary as gfi does not permit a branch to
 start from itself, and the branch is created in memory before the
-`from` command is even read from the input.  Adding `^0` will force
+`from` command is even read from the input.  Adding `{caret}0` will force
 gfi to resolve the commit through Git's revision parsing library,
 rather than its internal branch table, thereby loading in the
 existing value of the branch.