Fix awk script in filter list generation readme

Writing a file inplace does not work within an awk script. This CL
modifies the recommended script to write awk output to a temp file and
move it back to the original.

Change-Id: Ibc0e43d8b549a818fce2f4d48c557915275314eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090034
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748298}

components/subresource_filter/FILTER_LIST_GENERATION.md

@@ -91,7 +91,7 @@ An example using [EasyList](https://easylist.to/easylist/easylist.txt) follows:
 Appends whitelist rules and also deduplicates rules which only differ by their set of affected domains.
 ```sh
 1. grep ^@@ easylist.txt >> smaller_list.txt
-2. awk -F,domain= '{ if(!length($2)) table[$1] = ""; else table[$1 FS] = length(table[$1 FS]) ? table[$1 FS] "|" $2 : $2; } END{ for (key in table) print key table[key] }' smaller_list.txt > smaller_list.txt
+2. awk -F,domain= '{ if(!length($2)) table[$1] = ""; else table[$1 FS] = length(table[$1 FS]) ? table[$1 FS] "|" $2 : $2; } END{ for (key in table) print key table[key] }' smaller_list.txt > smaller_list_deduped.tmp && mv smaller_test_deduped.tmp smaller_list.txt
 3. sort smaller_list.txt | uniq > final_list.txt
 ```