AVRO-3957: [C] Fix typos in docs and examples (#2795)

* AVRO-3957: [C] Fix misformatted type in docs

Signed-off-by: Sahil Kang <sahil.kang@asilaycomputing.com>
Signed-off-by: Sahil Kang <sahilkang@google.com>

* AVRO-3957: [C] Fix type in quickstop example

This removes the following warning:

  avro/lang/c/examples/quickstop.c:123:40: warning: incompatible pointer types passing 'int32_t **' (aka 'int **') to parameter of type
        'const char **' [-Wincompatible-pointer-types]
                          avro_value_get_string(&first_value, &p, &size);

Signed-off-by: Sahil Kang <sahil.kang@asilaycomputing.com>
Signed-off-by: Sahil Kang <sahilkang@google.com>

---------

Signed-off-by: Sahil Kang <sahil.kang@asilaycomputing.com>
Signed-off-by: Sahil Kang <sahilkang@google.com>

lang/c/docs/index.txt

@@ -178,7 +178,7 @@ different versions of the Avro library.  That means that it's really
 only safe to use these hash values internally within the context of a
 single execution of a single application.
 
-The +reset+ method “clears out” an +avro_value_t instance, making sure
+The +reset+ method “clears out” an +avro_value_t+ instance, making sure
 that it's ready to accept the contents of a new value.  For scalars,
 this is usually a no-op, since the new value will just overwrite the old
 one.  For arrays and maps, this removes any existing elements from the

lang/c/examples/quickstop.c

@@ -107,7 +107,7 @@ int print_person(avro_file_reader_t db, avro_schema_t reader_schema)
 	if (rval == 0) {
 		int64_t id;
 		int32_t age;
-		int32_t *p;
+		const char *p;
 		size_t size;
 		avro_value_t id_value;
 		avro_value_t first_value;