python312Packages.pandas: remove leading `\` in postPatch

Makes adding a new command to `postPatch` not get interpreted as another
`substituteInPlace` argument.

pkgs/development/python-modules/pandas/default.nix

···
     postPatch = ''
       substituteInPlace pyproject.toml \
         --replace-fail "meson-python==0.13.1" "meson-python>=0.13.1" \
-        --replace-fail "meson==1.2.1" "meson>=1.2.1" \
+        --replace-fail "meson==1.2.1" "meson>=1.2.1"
     '';
 
     nativeBuildInputs = [