docs: fix unit in metrics semantic conventions example

Author: neoeinstein

opentelemetry-semantic-conventions/scripts/templates/registry/rust/metric.rs.j2

@@ -24,7 +24,7 @@
 //! let meter = global::meter("mylibraryname");
 //! let histogram = meter
 //!     .u64_histogram(semconv::metric::HTTP_SERVER_REQUEST_DURATION)
-//!     .with_unit("By")
+//!     .with_unit("s")
 //!     .with_description("Duration of HTTP server requests.")
 //!     .build();
 //! ```
@@ -76,4 +76,4 @@
 pub const {{ metric.metric_name | screaming_snake_case }}: &str = "{{ metric.metric_name }}";
 
     {% endfor %}
-{% endfor %}
+{% endfor %}