Add missing example parallelism value.

README.md

@@ -320,7 +320,7 @@ auth_opt_salt_size 16           # salt bytes length
 auth_opt_iterations 3           # number of iterations
 auth_opt_keylen 64              # key length
 auth_opt_memory 4096            # amount of memory (in kibibytes) to use
-auth_opt_parallelism            # degree of parallelism (i.e. number of threads)
+auth_opt_parallelism 2          # degree of parallelism (i.e. number of threads)
 ```
 
 **These options may be defined for each backend that needs a hasher by prepending the backend's name to the option, e.g. for setting `argon2id` as `Postgres'` hasher**:
@@ -353,7 +353,7 @@ If `log_dest` or `log_file` are invalid, or if there's an error opening the file
 
 #### Prefixes
 
-Though the plugin may have multiple backends enabled, there's a way to specify which backend must be used for a given user: prefixes. When enabled, `prefixes` allows to check if the username contains a predefined prefix in the form prefix_username and use the configured backend for that prefix. Options to enable and set prefixes are the following:
+Though the plugin may have multiple backends enabled, there's a way to specify which backend must be used for a given user: prefixes. When enabled, `prefixes` allow to check if the username contains a predefined prefix in the form prefix_username and use the configured backend for that prefix. Options to enable and set prefixes are the following:
 
 ```
 auth_opt_check_prefix true