man: Fix misspelled -fdebug-prefix-map flag in dpkg-buildflags(1)

Reported-by: Mattia Rizzolo <mattia@debian.org>

debian/changelog

@@ -18,6 +18,8 @@ dpkg (1.18.10) UNRELEASED; urgency=medium
     - Add references to deb-control(5) and deb-triggers(5) in dpkg(1).
     - Fix typos for repeated “the” word.
       Prompted by Valentin Samir <valentin.samir@crans.org> in #830989.
+    - Fix misspelled -fdebug-prefix-map flag in dpkg-buildflags(1).
+      Reported by Mattia Rizzolo <mattia@debian.org>.
 
   [ Updated programs translations ]
   * German (Sven Joachim).

man/dpkg-buildflags.1

@@ -412,7 +412,7 @@ This will cause warnings when the \fB__TIME__\fP, \fB__DATE__\fP and
 .TP
 .B fixdebugpath
 This setting (disabled by default) adds
-.BI \%\-fdebug\-path\-map= BUILDPATH =.
+.BI \%\-fdebug\-prefix\-map= BUILDPATH =.
 to \fBCFLAGS\fP, \fBCXXFLAGS\fP, \fBOBJCFLAGS\fP, \fBOBJCXXFLAGS\fP,
 \fBGCJFLAGS\fP, \fBFFLAGS\fP and \fBFCFLAGS\fP where \fBBUILDPATH\fP is
 set to the top-level directory of the package being built.