tree-wide: fix spelling of "fallback"

Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back"
is the verb. (This is pretty clear when we construct a sentence in the
present continous: "we are falling back" not "we are fallbacking").

src/shared/reboot-util.h

@@ -6,7 +6,7 @@ int update_reboot_parameter_and_warn(const char *parameter, bool keep);
 typedef enum RebootFlags {
         REBOOT_LOG      = 1 << 0, /* log about what we are going to do and all errors */
         REBOOT_DRY_RUN  = 1 << 1, /* return 0 right before actually doing the reboot */
-        REBOOT_FALLBACK = 1 << 2, /* fallback to plain reboot() if argument-based reboot doesn't work, isn't configured or doesn't apply otherwise */
+        REBOOT_FALLBACK = 1 << 2, /* fall back to plain reboot() if argument-based reboot doesn't work, isn't configured or doesn't apply otherwise */
 } RebootFlags;
 
 int read_reboot_parameter(char **parameter);