perf script: Fix documented const'ness of perf_dlfilter_fns

perf_dlfilter_fns must not be const, because it is not.

Declaring it const can result in it being mapped read-only, causing a
segfaullt when it is written. Update documentation accordingly.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: 8defa7147d5572 ("perf script Add API for filtering via dynamically loaded shared object")
Link: https //lore.kernel.org/r/20210811101036.17986-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-dlfilter.txt

@@ -32,7 +32,7 @@ The API for filtering consists of the following:
 ----
 #include <perf/perf_dlfilter.h>
 
-const struct perf_dlfilter_fns perf_dlfilter_fns;
+struct perf_dlfilter_fns perf_dlfilter_fns;
 
 int start(void **data, void *ctx);
 int stop(void *data, void *ctx);
@@ -214,7 +214,7 @@ Filter out everything except branches from "foo" to "bar":
 #include <perf/perf_dlfilter.h>
 #include <string.h>
 
-const struct perf_dlfilter_fns perf_dlfilter_fns;
+struct perf_dlfilter_fns perf_dlfilter_fns;
 
 int filter_event(void *data, const struct perf_dlfilter_sample *sample, void *ctx)
 {