Fix small errors in brew shellenv

These were introduced by typos and an incorrect assumption on the last
update of #15358. The last commit had been tested with a parameter, but
had not been re-tested *without* parameter.

Apologies for the error.

Library/Homebrew/brew.sh

@@ -81,6 +81,7 @@ case "$*" in
     ;;
   shellenv)
     source "${HOMEBREW_LIBRARY}/Homebrew/cmd/shellenv.sh"
+    shift
     homebrew-shellenv "$1"
     exit 0
     ;;

Library/Homebrew/cmd/shellenv.sh

@@ -24,7 +24,7 @@ homebrew-shellenv() {
   then
     HOMEBREW_SHELL_NAME="$1"
   else
-    HOMEBREW_SHELL_NAME="$(/bin/ps -p "${PPID}" -c -o comm=)}"
+    HOMEBREW_SHELL_NAME="$(/bin/ps -p "${PPID}" -c -o comm=)"
   fi
 
   case "${HOMEBREW_SHELL_NAME}" in