package/bash: add missing double-dollar for /etc/shells

Commit 4d279697afbf8fb295274784103be2b837113d5e missed one comment
from Yann E. Morin about the double-dollar.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/bash/bash.mk

@@ -45,7 +45,7 @@ define BASH_INSTALL_TARGET_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) \
 		DESTDIR=$(TARGET_DIR) exec_prefix=/ install
 	rm -f $(TARGET_DIR)/bin/bashbug
-	grep -qsE '^/bin/bash' $(TARGET_DIR)/etc/shells \
+	grep -qsE '^/bin/bash$$' $(TARGET_DIR)/etc/shells \
 		|| echo "/bin/bash" >> $(TARGET_DIR)/etc/shells
 endef