Make the :doc command work even if lf is not in the PATH

Previously, `:doc` executed `lf -doc`. Now, it executed `$lf -doc` or `%lf%
-doc`.

Before this change, `:doc` did not work if `lf` is not in the PATH. This is
likely to happen to new users who are trying out `lf` and need docs the most.

Moreover, if you made some changes to the docs and ran `lf` with `go run .`,
`:doc` would return the docs for the version of `lf` in the PATH instead of
what is expected.

os.go

@@ -159,7 +159,7 @@ func setDefaults() {
 	gOpts.keys["i"] = &execExpr{"$", `$PAGER "$f"`}
 	gOpts.keys["w"] = &execExpr{"$", "$SHELL"}
 
-	gOpts.cmds["doc"] = &execExpr{"$", "lf -doc | $PAGER"}
+	gOpts.cmds["doc"] = &execExpr{"$", `"$lf" -doc | $PAGER`}
 	gOpts.keys["<f-1>"] = &callExpr{"doc", nil, 1}
 }
 

os_windows.go

@@ -115,7 +115,7 @@ func setDefaults() {
 	gOpts.keys["i"] = &execExpr{"!", "%PAGER% %f%"}
 	gOpts.keys["w"] = &execExpr{"$", "%SHELL%"}
 
-	gOpts.cmds["doc"] = &execExpr{"!", "lf -doc | %PAGER%"}
+	gOpts.cmds["doc"] = &execExpr{"!", "%lf% -doc | %PAGER%"}
 	gOpts.keys["<f-1>"] = &callExpr{"doc", nil, 1}
 }