readme: Fix broken link to fuzzing report (#1573)

The fuzzing report link is broken in the readme. This change corrects
the relative URL.

Co-authored-by: Oliver Gould <ver@buoyant.io>
(cherry picked from commit 5cecf49)
Signed-off-by: Oliver Gould <ver@buoyant.io>

README.md

@@ -78,7 +78,7 @@ minutes to review our [code of conduct][coc].
 
 We test our code by way of fuzzing and this is described in [FUZZING.md](/docs/FUZZING.md).
 
-A third party security audit focused on fuzzing Linkerd2-proxy was performed by Ada Logics in 2021. The full report is available [here](docs/linkerd2-proxy-fuzzing-report.pdf).
+A third party security audit focused on fuzzing Linkerd2-proxy was performed by Ada Logics in 2021. The full report is available [here](/docs/reports/linkerd2-proxy-fuzzing-report.pdf).
 
 
 ## License