Revert "Fix error message mentionning graphicsmagick instead of imagemagick"

As mentioned in
[a followup comment](https://github.com/phiresky/ripgrep-all/pull/19#issuecomment-633287929)
when this change from "graphicsmagick" to "imagemagick" was originally
merged, the code does currently call `gm` (src/adapters/pdfpages.rs:62)
and so does, in fact, need the "graphicsmagick" package.

This reverts commit 6d2df9162c.

src/adapters/pdfpages.rs

@@ -73,7 +73,7 @@ impl FileAdapter for PdfPagesAdapter {
 
         let mut cmd = cmd
             .spawn()
-            .map_err(|e| map_exe_error(e, exe_name, "Make sure you have imagemagick installed."))?;
+            .map_err(|e| map_exe_error(e, exe_name, "Make sure you have graphicsmagick installed."))?;
         let args = config.args;
 
         let status = cmd.wait()?;